Field of the Invention
The present invention relates to a multi-feed detection apparatus for detecting multi-feed of sheets conveyed along a paper path, a sheet conveyance apparatus, and an image forming apparatus.
Description of the Related Art
Image forming apparatuses, e.g., printers, copiers, and multi-function devices, have a paper containing portion such as a paper cassette or a paper tray. Sheets of paper are stacked and held in the paper containing portion. When such an image forming apparatus is given a print job, sheets of paper are supplied, one by one, from the paper containing portion, and the image forming apparatus performs printing onto the sheets which are being conveyed, and outputs the sheets onto which printing has been carried out.
In general, upon determining that a trouble related to paper conveyance has occurred, the image forming apparatus stops the paper conveyance instantly, suspends the execution of the print job, and displays an error message to prompt a user to take some measures against the trouble. After that, the image forming apparatus waits for the user to remove the paper which remains in the paper path.
The trouble related to paper conveyance includes a trouble in which a plurality of sheets of paper partly overlapping each other along the conveyance direction is conveyed. Such a trouble is called “multi-feed”. The multi-feed occurs when pickup rollers and so on take the top sheet from the paper containing portion. The multi-feed occurs because a sheet of paper right down the topmost paper which is being taken out of the paper containing portion is dragged due to friction or electrostatic adsorption on the topmost paper.
In relation to multi-feed detection, a technology using an ultrasonic sensor has been proposed. According to the technology, a transmission part and a receiving part of the ultrasonic sensor are disposed in a paper path to face each other with paper passing therebetween. Whether multi-feed occurs or not is determined based on an ultrasonic wave received signal penetrating through the paper which is being conveyed (Japanese Unexamined Patent Application Publication No. 2011-157141).
Another technology for detecting multi-feed with a photo-interrupter has been proposed. According to the technology, whether multi-feed occurs or not is determined based on the amount of light received of detection light passing through the paper.
Another detection method has been proposed in which a displacement meter for detecting displacement of a lever with an encoder is used to detect, as multi-feed, a change in thickness of paper which is being conveyed.
As another conventional technology related to conveyance, a technology by a postal item processing device has been proposed. The device conveys a paper sheet with a pair of conveying belts and detects travel speeds of the conveying belts. When a difference between the travel speeds thereof is equal to or greater than a predetermined value, the device provides a notification (Japanese Unexamined Patent Application Publication No. 2014-159323). In order to detect the travel speeds, transparent holes are formed at a constant pitch along the conveyance direction. The detection light passing through the transparent holes are received to determine the travel speeds based on the period of light received and the pitch of the transparent holes.
For detection of multi-feed, the use of a contact sensor such as a displacement meter causes a problem that the paper face is scratched due to the contact by a lever. Such a scratch is conspicuous in, particularly, coated paper, e.g., gloss paper for photoprint, so that the quality of a printed matter is reduced.
The use of an ultrasonic sensor enables multi-feed detection in a non-contact manner with respect to a specific type (paper type) of paper of which an attenuation factor of ultrasonic wave transmitting the paper is known. However, the attenuation factor of ultrasonic wave differs depending on the paper type. This makes it difficult to detect multi-feed independently of the paper type. In particular, when paper used for printing is two-ply paper having a structure of two or more sheets overlapping each other, e.g., an envelope or a slip form, the ultrasonic sensor is not capable of distinguishing between multi-feed of sheets of normal paper and normal feed of such two-ply paper.
Further, in using the ultrasonic sensor, in order to prevent an erroneous detection due to influence of sneaking of ultrasonic wave, it is necessary to determine that multi-feed occurs when a state of received sound amount equal to or lower than a threshold continues for a predetermined time. Thus, unfortunately, it is impossible to detect multi-feed of sheets slightly overlapping each other.
In using a photo-interrupter for detection of multi-feed of sheets, there is a problem that paper usable for printing is limited only to translucent paper. When paper having a small light transmittance, e.g., thick paper or colored paper, is used for printing, the photo-interrupter is incapable of detecting multi-feed of sheets.